The present invention relates to a rotary basket air classifier.
Such a classifier or separator is known from EP No. 67,894 A 1. Such classifiers generally require dust separators, e.g. in the form of cyclones for separating the fine material particles from the classifying air. It can be appropriate for this purpose to remove the classifying air from the classifier as near as possible to the top, so that there is sufficient space for erecting the subsequently connected cyclone and the recirculating air blower which produces the classifying air. This is particularly important where the classifier is connected downstream of a mill and, to reduce costs, the classifier is to be as closely superimposed on the mill as possible.
This removal of the spent classifying air coaxially to the classifier rotation axis and as centrally as possible to the basket leads to important problems with respect to an optimum predispersion of the material to be classified. However, a particularly good predispersion is the prerequisite for an excellent selectivity during the classifying process. In other words, in an optimum-operating classifier, it is desirable that with a sought separation gain boundary as little oversize material as possible passes into the fines and simultaneously a minimum amount of undersize material passes into the coarse material. The particle mixture to be classified must therefore enter the classifying chamber in as uniformly as possible a distributed manner, whilst obviously preventing a dropping down of the particle mixture into the classifying chamber or counter to the whirling classifying air.
However, this necessary predispersion is not ensured in the case of a classifier known from EP No. 67,894 A 1, because around the upwardly directed suction line therein is provided a ring main for feeding in the particle mixture or raw material which is positioned directly above the classifying chamber, so that it can be assumed that the particle mixture to be classified in principle merely drops into the classifying chamber and even a classifying material entry counter to the whirling classifying air cannot be excluded.
In the case of another classifier known from EP No. 23,320 B 1, an attempt is made to obtain a scattering or strewing action for the introduced raw material or particle mixture in the case of a coaxially upwardly directed suction line for the fines-laden classifying air. However, a good dispersing action is not obtained, because in said known classifier merely the outer, flange-like edge of the upper termination of the basket can be used for a mechanical strewing means. In this known classifier, the particle mixture also drops substantially in punctiform manner through two or more inlets arranged around the central suction line onto the flange-like edge rotating at a relatively high circumferential speed. Each of the corresponding impact points radially accelerates the individual particles of the screening material in an immediate manner and in the case of a limited drag only a triangular strewing mist can form, which can be interrupted by adjacent strewing mists. The further disadvantage exists in this known classifier that in the case of varying speeds of the basket relative to varying separating particle boundaries, the shape and spacings of the strewing mists can change, so that it is not possible to obtain a uniform distribution of the material to be classified. The lack of a central classifying material feed, linked with the non-uniform distribution action of the flange-like edge, has a particularly unfavourable effect on the sought optimum dispersing process.
As a good predispersion helps significantly to determine the throughput capacities for the material to be classified, this being of particular importance in the case of a preceding grinding in the closed grinding material circuit, as quantity fluctuations there can have a sudden effect on the classifying process, the prior art constantly attempts to obtain a good predispersion.
Thus, e.g. a PSZ spiral air classifier is known (Zement-Kalk-Gips, Vol. 38, no. 1/85 "Neue Erkenntnisse zur Sichtergestaltung", by F. Sgaslik, FIG. 7) in which a predispersion of the material to be classified is sought. The material to be classified is fed centrally into this PSZ classifier and is strewn by a sloping, rotating strewing plate or disk into a downwardly widened, conical distribution gap. Admittedly, the uniformity of the classifying material distribution improves with increasing diameter. However, experience has shown that the angle of inclination of this distribution gap surrounding the classifying chamber is dependent on the friction coefficients of the friable or trickling particle mixture. In the case of an optimum classifying material distribution by trickling, the particle mixture must not drop and must instead just be able to slide, i.e. it must be flowable. In addition, the trickling path must be long enough to achieve the desired predispersion. In order in the case of this known PSZ classifier to obtain a wide region for varyingly friable classifying material the complicated procedure is used therein of blowing scavenging air into the conical distribution gap. Moreover, in the case of this known PSZ classifier the problem occurs that it is necessary to accept a large horizontal component for a sufficiently long trickling path, the flow-correct distance from the basket to the angular momentum-producing guide vanes being exceeded and consequently the circular classifying chamber is enlarged. The resulting limited curvature of the necessary classifying air spirals in the classifying chamber makes it necessary to operate with higher classifying air velocities for maintaining the same classifying effect. However, this means that for larger air quantities correspondingly larger deposition means, such as cyclone separators or filters are required, which considerably increase equipment costs.